>> tom foreman, thanks for that explanation. our own pentagon correspondent, chris lawrence, went over the videos of this season with a former atf assistant director michael bouchard. >> can they take that video from the second scene and start to build the case backwards to photos like these? >> yes. what the investigators will do will look at the cameras an hour and a half, two hours before the bomb blasts to see who was in the area at the time. checking it every 15, 20 minutes. if they see the same person in the same pictures, it's not likely they were there to watch someone else finish the race. >> can we tell pretty much where that first bomb was plant? >> yes, in this picture between the light pole and the building, you can see how heavily concentrated the people are. so if someone left a backpack or duffel bag on the ground, people wouldn't really notice it. >> is there any way the bomb could have been placed on the curb? >> it's highly unlikely the bomb was placed on the curb. because when the bomb went off, this were run